//vulcan.wr.usgs.gov/photo/slideset

Mount St Helens, in Washington State, USA, is one of the Cascade Range of volcanoes, a chain stretching from northern California to British Columbia.

Beaumont School Natural Hazards: Mount St Helens Eruption: May

18, 1980

The smallest of five major volcanic peaks was Mount St Helens with an elevation of 2,950m.

Set in one of America’s most spectacular national forests, before the eruption Mount St Helens was one of the most frequently climbed and considered one of the most beautiful peaks in the Cascade Range. Spirit Lake to the north, above, was part of a holiday area offering hiking, camping, boating and fishing.

On March 20, 1980, the first seismic twitch, a magnitude 4 earthquake, indicated that Mount St Helens was resuming activity after a dormant period of 123 years. Swarms of earthquakes and small ash and gas eruptions suggested an eruption could follow.

On March 27, another earthquake led to a fresh 65m-wide crater opening on the summit. Seismic events continued into April, above.

Vulcanologists from the United States Geological Survey (USGS) were pressed for predictions. Ash samples were tested after each mini release of steam, and checked for sulphur dioxide levels – a rise would indicate that magma was on the move. Volcanic gas collections were taken via aeroplane.

The cause of the activity was the subduction of the Juan de Fuca plate beneath the North American plate.

Cross-section through the Cascade Range subduction zone. Scientists predicted the volcano could erupt at any time and, in response, a five-mile exclusions area was made for tourists and sightseers.

A developing bulge was detected by tiltmeters on the volcano’s north flank, and it soon became clear to the naked eye that the volcano’s north face was swelling ominously. The bulge, right, developed as magma pushed up within the peak. Angle and slope-distance measurements indicated it was growing at a rate of up to 1.5m per day. It had pushed upwards and outwards 100m by mid April, and was over 135m by May 17.

Satellite images can record the warming of the ground surface as the magma edges towards the breakthough point. This satellite image of Mount St Helens was taken about one month before it erupted. The infra-red image shows the hottest areas of the volcano in red and the coolest in black.

The explosion took place following a long sequence of events. The previous eruption, in 1857, had created weaknesses within the volcanic cone. 123 years later, rising magma and gas changed the shape of the volcano as it began to bulge.

On May 18 at 8:32am (Pacific Daylight Time), an earthquake of Richter Scale magnitude 5.1, caused by an injection of magma under the volcano’s north flank, triggered the biggest landslide ever recorded.

A mixture of rock, glacier ice and soil flowed as lahars into Spirit Lake, causing waves 200m high, then down the North Fork of the Toutle River, filling the valley with 100m of debris.

By early the next day, the devastating eruption was over. The blast, which overtook the avalanche, caused many of the 57 fatalities that occurred as the combined result of the eruption and pyroclastic injuries, burns and the inhalation of poisonous ash and gases.

Many of the dead had managed to gain access to restricted areas to obtain a better view of the volcano. Some were scientists monitoring the volcano's activity. An estimated 24,000 animals perished.

As the avalanche moved away from Mount St Helens, the huge reduction of pressure caused an enormous lateral blast of hot ash, pumice and gas. The blast was followed by pyroclastic flows pouring out of the crater which, at temperatures of over 3000C, travelled northwards at speeds of over 400km/h.

Spirit Lake

The area north of the volcano bore the brunt of the lateral blast deposits. Although the eruption was predicted, its scale and timing surprised the United States Geological Survey scientists. As the blast was mainly directed northwards, debris was deposited beyond the evacuation zone in this direction.

An estimated 150 million cubic metres of material was deposited directly by lahars (volcanic mudflows) into river channels. The lahars were responsible for most of the property damage that occurred as roads, houses and bridges in the Toutle River valleys were destroyed.

Given the size of the event, the number of deaths was relatively small. This was partly due to the evacuation zone, but also because as the blast occurred on a Sunday, many of the people who worked in forestry were not in the area.

An area of over 500 square kms was scorched. Up to 32km away full grown Douglas Fir trees, right, were stripped clean and combed down flat against the hills.

The slopes of Smith Creek valley, east of Mount St Helens, left. The direction of the blast, shown here from left to right, is apparent in the alignment of the trees. Enough timber to build 150,000 homes was damaged or destroyed.

The debris avalanche from Mount St Helens covered over 62 square kms of the upper Toutle River valley and blocked tributaries of the North Fork Toutle River. New lakes such as Castle Lake, right, and Coldwater Lake were created.

One major concern to people living downstream of Mount St Helens was a breakout of any of these impounded lakes due to the instability of the debris dams blocking them. Flood waters from a breakout could be more catastrophic than the lahars of the eruption. Gauges were installed at lakes and streams surrounding the volcano, left, to monitor changes in water levels.

Another major problem to people living downstream of the volcano was the high sedimentation rates resulting from stream erosion of the volcanic deposits. Avalanche and lahar deposits were transported downstream and deposited on channel beds, decreasing the carrying capacity of the channels and increasing the chances of floods.

Sediment eventually found its way into the Columbia River reducing the size of the shipping canal, which subsequently had to be dredged.

Nearly 135 miles of river channels surrounding the volcano were affected by the lahars of May 18. A mudline left behind on trees shows depths reached by the mud. A scientist (middle right) gives scale.

For more than nine hours on May 18 a vigorous plume of ash erupted and spread north-eastwards, eventually reaching 30km into the sky. 540 million tonnes of ash were ejected into the atmosphere.

Noticeable ash fell in 11 American states. Just three days after the eruption, air pollution monitoring systems detected ash in east coast cities such as New York, some 4,000km away. The ash circled the globe in 17 days.

Ash Impacts

• People across north-western America were told to stay indoors and wear gauze masks.

• The ash made roads slippery and reduced visibility. Many roads were closed, trains halted and aircraft grounded.

• Fish, in some hatcheries, perished as ash fell into lakes and streams, clogging their gills and raising the temperature of the water.

• Crops were destroyed, or subsequently produced low yields, because ash settled on leaves, impeding photosynthesis.

• Electricity supplies were interrupted and sewers were blocked, and the ash damaged car engines.

• The town of Yakima, as an example, some 150km away, was blanketed in 1.5cm of ash. Skies were turned grey as far away as as Spokane (see previous slide), Washington State, 400km away.

By the Monday morning of May 19 following Sunday’s eruption, Mount St Helens' elevation was only 2,550m (from 2,950m) and it had a 1.5km-wide, horseshoe-shaped crater, above.

Mount St Helens continued to erupt over subsequent years. An explosive eruption on March 19, 1982, resulted in a lahar (the dark deposit on the snow, left) flowing from the crater into the North Fork Toutle River valley.

Five more eruptions occurred during 1980, including an event on July 22 which sent pumice and ash 6 to 11 miles into the air, right.

The dome was blown to pieces during the 3-hour eruption. A big increase in sulphur dioxide had been recorded by USGS scientists three days earlier.

Scientists learned a great deal about volcanoes from the explosion and have been studying how plant life has gradually returned to the once devastated region around the mountain. Today, the foothills are dotted with wild flowers and young pine, alder and aspen trees. Since the eruption, the volcano has been rebuilding a new dome of silica-rich acidic magma and has changed out of all recognition.

The volcano is now within the Mount St Helens National Volcanic Monument. The US Forest Service did not officially re-open the mountain to climbers until May 18, 1987. Visitor centres and trails were established as tourists, students and scientists visited. Every year nearly 2 million visit.

Will Mount Rainier be next?

Mount RainierAt least half the volcanoes in the Cascade Range still active and there are fears for future eruptions.

Of particular concern is Mount Rainier, which is now considered the most dangerous volcano in America. At 4,392 meters, it is the highest peak in the Cascade Range, and encased in over 35 square miles of snow and ice. It is only about 60 miles from the built-up area of Seattle.

Geological evidence suggests thousands of people live in areas cover by its historic mudflows, some of which have been 60 miles long.

People at the base of the volcano, at towns such as Orting, are at particular risk, and here schools practice evacuation drills. 

END

Last super-eruption 74,000 years ago on Indonesian island of Toba, created global volcanic catastrope. Human pop from hundreds of thousands to pop of less than 10,000, poss from global effects of eruption. Occur twice every 100,000 years, so are we due another?

Biggest eruption could release 2000 cubic miles into atmosphere, creating ice age and years without sunlight. Super-eruptions are normal part of earth’s workings, they are not going to stop just because we have appeared.

• One of the dead was local celebrity 84-year-old Harry Truman. The former president's namesake had consistently refused to leave his motel at the foot of the mountain beside Spirit Lake. He stayed while about 2,000 people were evacuated from the area in the last few weeks as the mountain threatened to blow. His body was never found.